1. Field of the Invention
The present application relates generally to surface emitting lasers, and more particularly to extended cavity surface emitting lasers.
2. Description of the Related Art
An individual VECSEL includes a gain element, which is typically a semiconductor die that includes a quantum well gain region and at least one distributed Bragg reflector (DBR). An individual gain element is also fashioned to restrict current injection to a desired region about the quantum well gain region. Spaced apart from the gain element is an element that functions as a mirror and which defines an “extended cavity.” Thus, an individual single emitter VECSEL has one gain element disposed within a single VECSEL laser cavity. A nonlinear optical crystal is often included in the extended cavity between the gain element and the mirror to generate frequency doubled light from a fundamental frequency of the VECSEL via intra-cavity second harmonic generation (SHG).
There are a number of limitations of a conventional single emitter VECSEL cavity design. One drawback is that the output power is typically thermally limited. Waste heat from current injection causes the temperature of the gain element to rise. At high currents the die temperature becomes too hot for efficient operation.
One technique that is used to increase the output power is to increase the emitting aperture of the VECSEL. However, there are limits on how much the output power can be increased by increasing the emitting aperture. That is, it is not possible to arbitrarily increase the size of the emitting aperture of a VECSEL to increase power. Transverse mode instabilities tend to set in at large apertures. The gradients in optical attributes across the emitter surface of the VECSEL also tend to become too large for stable operation beyond a certain aperture size. The SHG efficiency also drops, because the fundamental beam waist becomes too large.
One approach that has been attempted to address the limitations of single emitter VECSELs is a parallel architecture. FIG. 1 illustrates a parallel architecture array of surface emitting lasers, which is described in more detail in U.S. Ser. No. 11/193,317, the contents of which are hereby incorporated by reference. An individual laser array is composed of a monolithic planar array of VECSEL surface emitting emitters formed on one die 105. Thus, all of the individual lasers in the array will be formed with similar quantum well active region layers and DBR layers. Semiconductor fabrication techniques are used to restrict the flow of current to define an array of pumped gain regions in the die. A common external optic 137, which could be reflective, diffractive or refractive, is spaced apart from the die. By way of a non-limiting example, the optic 137 can be a Volume Bragg Grating or VBG, which provides spectrally narrow reflectivity. Note that this external optic defines parallel extended cavities and the individual lasers are spaced far enough apart that all of the lasers in the array operate in parallel. In frequency shifted applications, a common nonlinear, converter 135, such as second harmonic generator, may be included in the extended cavity for all emitters of the array.
A parallel VECSEL architecture has an output power that theoretically scales with the number of individual emitters. That is, at best, a parallel VECSEL architecture increases output power by the single emitter output multiplied by the number of emitters. However, practical considerations make it difficult to achieve a true scaling of output power. One effect that limits the capabilities of a parallel VECSEL architecture is that in practice it is difficult to simultaneously optimize performance of all the emitters in a larger array, which reduces output power. One reason for this is that the VESCEL array must be very flat for all the parallel cavities to be aligned simultaneously using a single external optic to provide the cavity feedback. However, as the lateral dimensions (e.g. width) of the array are increased, the small amounts of curvature that are naturally present in the semiconductor structure due to dissimilar material properties through the device and packaging structure mean that the necessary alignment becomes harder to achieve, thus reducing the optical power yield for large area arrays, i.e. for a given radius of curvature of the array die within the package, the angular and lateral misalignments of the optical path at the outer ends of the array increase as the size of the array increases. Eventually, as the array size and misalignments increase, the optical losses for the outer elements becomes high enough to significantly reduce their lasing efficiency, or even prevent their lasing altogether. Since the cavity feedback can be provided (for reasons of cost alignment simplicity) by the single common external optic, the alignment of the optic cannot be adjusted independently across the array to optimize individual emitters to correct for the laser die curvature.
The VECSEL die size is also constrained by mechanical considerations. The die size cannot be arbitrarily large because mechanical fragility will compromise manufacturability. This is true both of the handling and processing of the bare die, and due to the effects of thermal expansion mismatch in the mounting of the bare die to a carrier or the submount/heatsink within the package. Since it is in general impossible to provide a perfect thermal expansion match between the VECSEL die and the carrier or submount, as the die size is increased the linear expansion difference between the components during the fabrication processing and laser operation is increased. Thus, as the die size is increased the stresses applied to the die are also increased and eventually this can lead to cracking and failure of the die, especially if the packaged die is subjected to multiple thermal cycling events, as can occur when the laser system is turned on and off. Thermal expansion mismatch of the die and carrier or submount can also result in curvature of the die, which can provide performance variations over temperature. The use of “hard” solders, such as gold-tin, to perform the bonding of the die to the carrier or submount, which are desirable for long term reliability due to their chemical inertness and resistance to voiding and creep, effectively prevents any strain relief between the die and carrier, magnifying the mechanical effects of thermal expansion mismatch. (the use of “soft” e.g. indium solders for the mounting of the die does allow some strain relief, but these solders are less stable and more subject to property changes and creep during operation at the high temperature interface between the die and the carrier/submount.)
Thermal effects also limit the total power that can be produced by a VECSEL array. As the array size is increased it becomes harder to extract the waste heat generated in the semiconductor material efficiently from under the relatively small footprint of the array. Due to these heat extraction limitations there is thermal cross-talk between the emitters in the array, such that the average temperature of the emitters in the array is increased compared to the temperature of a single emitter which is pumped at the same electrical drive levels. It should be noted that these heat extraction limitations are exacerbated by the increase in die size. As the die size is increased the mechanical expansion effects described above force the use of thermally expansion matched carriers or submounts to attach the die. In general, the materials that provide the best expansion match to the die do not necessarily provide the best thermal conductivity and heat removal. For instance, diamond provides very high heat conductivity and allows the laser emitters to operate at a lower temperature for a given drive current, therefore maximizing output power and efficiency, but the thermal expansion mismatch is too high to allow bonding with hard solders.
Another drawback of a VECSEL array is that the system efficiency is inherently low. A harmonic converter needs high power at the fundamental frequency to achieve efficient second harmonic conversion of light at the fundamental frequency to the second harmonic. However, it can be difficult to achieve a high power at the fundamental frequency in a VECSEL. Principally, the gain in a VECSEL emitter is inherently low because the gain region is very thin, comprising only the thickness of the epitaxially grown quantum wells, and thus the single pass gain is typically only a few percent. Since a number of optical elements are required to form the external cavity for the frequency doubled VECSEL a number of inherent losses are added to the cavity which degrade the laser performance. Although all the cavity elements can be optimized using anti-reflection coatings and high reflectivity coatings where necessary, there are always some residual reflection and transmission losses from the optical coatings, and some small absorption losses related to transmission through some of the components. Overcoming these cavity losses with the relatively low gain VECSEL emitter limits the attainable output power and the efficiency of the laser system. Thus the opportunity to minimize the number of cavity components required per VECSEL gain element is desirable for improving the laser system output power and efficiency.
Additionally, there are practical limitations on the output mode quality in a parallel architecture. The VECSEL array functions as an array of independent lasers with as many beams as emitters. As a result, if the output power of the laser system is scaled by increasing the lateral dimensions of the array, the output beams are spread over a larger area and the output of the VECSEL array is more difficult to focus to a small spot.